A longitudinal study of the changes in the static accommodation response

BCLA CLEAR presbyopia: Mechanism and optics

With over a billion adults worldwide currently affected, presbyopia remains a ubiquitous, global problem. Despite over a century of study, the precise mechanism of ocular accommodation and presbyopia progression remains a topic of debate. Accordingly, this narrative review outlines the lenticular and extralenticular components of accommodation together with the impact of age on the accommodative apparatus, neural control of accommodation, models of accommodation, the impact of presbyopia on retinal image quality, and both historic and contemporary theories of presbyopia.

BCLA CLEAR Presbyopia: Evaluation and diagnosis

It is important to be able to measure the range of clear focus in clinical practice to advise on presbyopia correction techniques and to optimise the correction power. Both subjective and objective techniques are necessary: subjective techniques (such as patient reported outcome questionnaires and defocus curves) assess the impact of presbyopia on a patient and how the combination of residual objective accommodation and their natural DoF work for them; objective techniques (such as autorefraction, corneal topography and lens imaging) allow the clinician to understand how well a technique is working optically and whether it is the right choice or how adjustments can be made to optimise performance. Techniques to assess visual performance and adverse effects must be carefully conducted to gain a reliable end-point, considering the target size, contrast and illumination. Objective techniques are generally more reliable, can help to explain unexpected subjective results and imaging can be a powerful communication tool with patients. A clear diagnosis, excluding factors such as binocular vision issues or digital eye strain that can also cause similar symptoms, is critical for the patient to understand and adapt to presbyopia. Some corrective options are more permanent, such as implanted inlays / intraocular lenses or laser refractive surgery, so the optics can be trialled with contact lenses in advance (including differences between the eyes) to better communicate with the patient how the optics will work for them so they can make an informed choice.

The effect of multifocal contact lenses on the dynamic accommodation step response

PURPOSE

To measure the dynamic accommodation response (AR) to step stimuli with and without multifocal contact lenses (MFCLs), in emmetropes and myopes.

METHODS

Twenty-two adult subjects viewed alternating distance (0.25D) and near (3D) Maltese crosses placed in free space, through two contact lens types: single vision (SVCL) or centre-distance multifocal (MFCL; +2.50D add). The AR level was measured along with near to far (N-F) and far to near (F-N) step response characteristics: percentage of correct responses, magnitude, latency, peak velocity and duration of step response.

RESULTS

There was no difference between N-F and F-N responses, or between refractive groups in any aspect of the accommodation step response dynamics. The percentage of correct responses was unaffected by contact lens type. Through MFCLs, subjects demonstrated smaller magnitude, longer latency, shorter duration and slower peak velocity steps than through SVCLs. When viewing the near target, the AR through MFCLs was significantly lower than through SVCLs. When viewing the distance target with the MFCL, the focal points from rays travelling through the distance and near zones were approximately 0.004D behind and 2.50D in front of the retina, respectively. When viewing the near target, the respective values were approximately 1.89D behind and 0.61D in front of the retina.

CONCLUSION

The defocus error required for accommodation control appears not to be solely derived from the distance zone of the MFCL. This results in reduced performance in response to abruptly changing vergence stimuli; however, these errors were small and unlikely to impact everyday visual tasks. There was a decrease in ocular accommodation during near tasks, which has previously been correlated with a reduced myopic treatment response through these lenses. With MFCLs, the estimated dioptric myopic defocus was the largest when viewing a distant stimulus, supporting the hypothesis that the outdoors provides a beneficial visual environment to reduce myopia progression.

Longitudinal changes in objective accommodative response, pupil size and spherical aberration: A case study

PURPOSE

Previous transverse and a handful of longitudinal studies have shown that the slope of the static accommodation response/stimulus curve declines as complete presbyopia is approached. Changes in pupillary miosis and ocular spherical aberration (SA) are also evident. This study further investigated longitudinal changes in the relationships between the monocular static accommodative response, pupil diameter and SA of a single adult.

METHODS

A wavefront analysing system, the Complete Ophthalmic Analysis System, was used in conjunction with a Badal optometer to allow continuous recording of the aberration structure of the dominant eye in a low myope for a range of accommodative demands (-0.83 to 7.63 D) over a period of 17 years until the age of 50. Monocular accommodative response was calculated as the equivalent refraction minimising wavefront error. The associated longitudinal changes in pupil size and SA with accommodation were also recorded.

RESULTS

A decrease in accommodation response with age was found at almost all target vergences, with the changes being greatest for higher vergences. In addition, although absolute pupil diameter decreased with age, the rate of change in pupil diameter with accommodative stimulus remained approximately constant with age. Pupil constriction occurred for near stimuli even in full presbyopia. SA changed linearly with the accommodation response at all ages.

CONCLUSIONS

The objective amplitude of accommodation declined linearly with age as complete presbyopia was approached, while the slope of the response/stimulus curve also fell. It was hypothesised that the retinal image blur associated with the larger lags of accommodation at higher accommodative stimuli was reduced by pupil constriction and the resulting lower levels of SA.

Comparison of the Amplitude of Accommodation Measured Using a New-Generation Closed-Field Autorefractor with Conventional Subjective Methods

Purpose: This study aims to compare and assess the agreement of the objective amplitude of accommodation (AA) measured using a new-generation closed-field autorefractor with conventional subjective methods. Methods: In total, 84 healthy individuals with an age range of 19 to 50 years participated in this cross-sectional study. AA was measured objectively with a Nidek autorefractor (AR-1a; Nidek Co., Ltd., Tokyo, Japan) and subjectively using push-up (PU) and minus-lens (ML) methods in a random order. Comparison between different methods was performed using repeated-measures analysis of variance and the Bonferroni test for pairwise comparisons. In addition to the Pearson correlation, the Bland and Altman method and the intraclass correlation coefficient were used to determine the agreement between the three techniques. Only the right-eye results were used for analysis. Results: AA measured using the Nidek autorefractor (3.43 ± 1.94 D) was significantly lower than that measured with PU (7.67 ± 2.38 D; p < 0.001) and ML (7.60 ± 2.81 D; p < 0.001) methods. The difference between the subjective methods was not significant statistically (p = 1.0). The correlation for Nidek measurements and PU and ML methods was moderate (r = 0.5502 and r = 0.6832, respectively), while it was strong when comparing subjective methods (r = 0.7821). The limits of agreement for Nidek vs. PU, Nidek vs. ML, and PU vs. ML methods were −8.28 to −0.23 D, −8.19 to −0.15 D, and −3.38 to 3.51 D, respectively. Conclusions: There was a moderate agreement between AA obtained with subjective methods and objective Nidek measurements. The objective AA measurements obtained with a new Nidek autorefractor were significantly lower than subjective measurements.

Prescription of near addition and its relation to accommodative reserve in presbyopia - The dichotomy between theory and practice

Purpose:

To study the relationship between the amplitude of accommodation (AA) measured by the Royal Air Force (RAF) rule and near addition (NA) prescribed in presbyopic patients to assess how far practice is different from theoretical recommendations.

Methods:

Patients, aged 40 to 60 years with best corrected vision of 6/6 N5 were included in this cross sectional observational study. AA was measured using the RAF rule. Refined with near Duochrome chart, the NA was given with the chart at 40 cm. Accommodative reserve was calculated from the measured AA, NA required and the calculated additional accommodation expended by the patient for reading. The percentage of reserve was calculated with and without the correction for depth-of-field.

Results:

A total of 130 patients were studied. In the patients above 50 years of age, the negative correlation of AA with age was - 0.298 (P = 0.065) and AA and NA was - 0.365 (P = 0.002) was weak. Among the patients aged between 40 and 50 years, the negative correlation of AA with Age and NA were strong; - 0.853 (P < 0.0001) and - 0.823 (P < 0.0001) respectively. When prescribing glasses, 62.01% (CI 58.78 to 65.23) and 90.93% (CI 62.50 to 119.37) of the AA was found to be kept in reserve for patients below and above 50 years of age respectively.

Conclusion:

Accommodative reserve kept in our study patients when prescribing NA was two to three times the theoretical recommendation. High percentage of accommodation kept in reserve suggest that the measured AA is not activated normally and require higher NA and one should not rely on patient’s ability to generate accommodative power especially in patients over 50 years.

Accommodation and age-dependent eye model based on in vivo measurements

PURPOSE

To develop a flexible model of the average eye that incorporates changes with age and accommodation in all optical parameters, including entrance pupil diameter, under photopic, natural, environmental conditions.

METHODS

We collated retrospective in vivo measurements of all optical parameters, including entrance pupil diameter. Ray-tracing was used to calculate the wavefront aberrations of the eye model as a function of age, stimulus vergence and pupil diameter. These aberrations were used to calculate objective refraction using paraxial curvature matching. This was also done for several stimulus positions to calculate the accommodation response/stimulus curve.

RESULTS

The model predicts a hyperopic change in distance refraction as the eye ages (+0.22D every 10 years) between 20 and 65 years. The slope of the accommodation response/stimulus curve was 0.72 for a 25 years-old subject, with little change between 20 and 45 years. A trend to a more negative value of primary spherical aberration as the eye accommodates is predicted for all ages (20-50 years). When accommodation is relaxed, a slight increase in primary spherical aberration (0.008μm every 10 years) between 20 and 65 years is predicted, for an age-dependent entrance pupil diameter ranging between 3.58mm (20 years) and 3.05mm (65 years). Results match reasonably well with studies performed in real eyes, except that spherical aberration is systematically slightly negative as compared with the practical data.

CONCLUSIONS

The proposed eye model is able to predict changes in objective refraction and accommodation response. It has the potential to be a useful design and testing tool for devices (e.g. intraocular lenses or contact lenses) designed to correct the eye's optical errors.

Accommodation in Early Presbyopes Fit with Bilateral or Unilateral Near Add

SIGNIFICANCE

When fit with monovision, most early presbyopes (aged 40 to 50 years) accommodated to near objects by focusing the distance corrected eye, leaving the near corrected eye myopically defocused with reduced image quality. A few were able to switch focus to the near corrected eye retaining a consistently focused image in one eye over a wider range of distances.

PURPOSE

The aim of this study was to examine accommodation behavior, pupil responses, and resultant image quality of early presbyopes fit with either bilateral or unilateral (monovision) near adds.

METHODS

Accommodative response and pupil size of 19 subjects (27 to 60 years), including 13 early presbyopes (40 to 50 years), were measured using an aberrometer as a binocularly viewed 20/40 letter E was moved from 2 m to 20 cm. Each subject was fit with different refractive strategies: bilateral distance correction, bilateral +2 diopters (D) near add, and unilateral +2 D near add placed over the measured right eye or unmeasured left eye. Monochromatic image quality was quantified using the Visual Strehl ratio metric.

RESULTS

With bilateral +2 D near add, all early presbyopes mostly refrained from accommodating (gain = 0.22 D/D) until the target approached closer than the 50-cm far point, and they then accommodated accurately until their maximum accommodative amplitude was reached. With monovision, most (10 of 13 early presbyopes) accommodated to focus the distance corrected eye, leaving the near corrected eye myopically defocused with reduced image quality. As stimulus distance became closer than their distance corrected eye's near point, they continued to exert maximum accommodation. Only two early presbyopes relaxed their accommodation to "switch" focus to the near corrected eye as target distance was reduced, and these two did not experience bilateral drop in image quality as stimulus distance became closer than the near point of the distance corrected eye.

CONCLUSIONS

Our data suggest that many early presbyopes will not initially adopt an accommodation strategy that optimizes image quality with monovision, but consistently accommodate to focus the distance corrected eye.

The distribution of negative and positive relative accommodation and their relationship with binocular and refractive indices in a young population

PURPOSE

To determine the distribution of negative relative accommodation (NRA) and positive relative accommodation (PRA) and its relationship with binocular vision indices in a young population.

METHODS

In this cross-sectional study conducted in a student population, samples were selected through multistage cluster sampling. All the samples underwent the measurement of uncorrected and corrected visual acuity and refraction. Then far and near cover tests were performed. The near point of convergence (NPC) and accommodation, accommodation facility, PRA and NRA were evaluated in all participants.

RESULTS

The mean age of the 382 participants was 22.5 ± 4.4 years (18-35 years). Mean NRA and PRA in the total sample was +2.08 ± 0.33 diopter (D) and -2.92 ± 0.76 D, respectively. Mean NRA was highest in hyperopic (P = 0.002) and mean PRA was highest in myopic (P = 0.003) participants. The multiple model showed that NRA had a direct relationship with accommodation facility and spherical refractive error, while PRA had a direct relationship with amplitude of accommodation (AA).

CONCLUSION

This study provides the normal range of the NRA and PRA and their relationship with accommodation facility, spherical refractive error, and AA in a sample of the Iranian population.

Accommodative Gain in Relation to Perceived Target Clarity

PURPOSE

The purpose of this study is to determine the relationship between target clarity and the magnitude of accommodative lag using the metric accommodative gain (AG).

METHODS

Monocular accommodative responses were measured with Grand Seiko autorefraction using both proximal and minus lens techniques in 139 subjects aged 5 to 35 years. Subjects viewed a 1.5-mm letter at 13 discrete distances (range, 40 to 3.33 cm) for the proximal technique and fixed at 33 cm through minus lenses of increasing power for the lens technique. Subjects were instructed to keep the target clear and report when it blurred. The AG was calculated (accommodative response/accommodative demand) for the four greatest consecutive demands perceived clear (termed conditions 1 to 4) and the first demand perceived blurry (termed condition 5).

RESULTS

Multivariate planned contrast, including age as a predictor, revealed that mean AG was significantly larger when the target was clear (range, 0.71 to 0.77 for conditions 1 to 4 across techniques) versus blurry (0.59 and 0.68 for condition 5 across techniques) (p < 0.001 for proximal and p < 0.036 for lens). Age was only a contributing factor for the proximal technique, with the youngest subjects having the largest decrease in AG when the target changed from clear to blurry (p = 0.017).

CONCLUSIONS

These data suggest that across age and technique, the AG is relatively constant when the target is perceived clear but drops below approximately 70%, on average, once the target is perceived as blurry for subjects aged 5 to 35 years. The AG may be a useful metric to compare accommodative responses across a range of demands and to identify accommodative responses that may not be sufficient to perceive a clear target.

Age-related changes in accommodation measured with a double-pass system

PURPOSE

To measure the accommodative response in a wide age range population using the retinal image quality assessments provided by a double-pass system.

METHODS

The accommodative response was measured using a custom-built double-pass setup in 84 patients from 15 to 55 years of age. Patients were classified in four groups (A: 15-25 year old; B: 26-35; C: 36-45; D: 46-55). Accommodation was stimulated from 0 to 5 dioptres (D) with the push up method using an open field fixation test. The total accommodative response in the stimulated range, the accommodative stimulus-response curve, the slope and the progression of optical quality with accommodation were measured.

RESULTS

The total accommodation obtained in groups A and B was high, with a mean value of 4 D or higher, whereas values in older individuals were significantly lower. The accommodative stimulus-response curve and the slope were also high with a sudden decrease in patients over 35 years of age. The optical quality of the eye during accommodation did not change significantly.

CONCLUSIONS

Participants aged 15-35 years showed a good accommodative response. Thereafter, a significant decrease of the response in the total accommodation, stimulus response curve and slope was observed. The younger groups showed a larger accommodative response than previously published.

Visual performance with simultaneous vision multifocal contact lenses

PURPOSE

  The aim was to assess visual performance after PureVision high addition multifocal contact lens (PM) fitting through contrast sensitivity measured under different illumination levels for both distance and near.

METHODS

  In a cross-over study, 20 presbyopic patients were randomly assigned to PM or spectacles. Near contrast sensitivity was measured with the Vistech VCTS 6500 system. Distance contrast sensitivity and distance visual acuity (VA) were measured with the Functional Vision Analyser under photopic (85 cd/m(2) ) and mesopic (3.0 cd/m(2) ) conditions, the latter also under two levels of induced glare: 1.0 lux and 28 lux. Near VA was measured by logMAR charts under approximately 80 cd/m(2) luminance settings.

RESULTS

  Average monocular spectacle corrected and PM VA were -0.02 ± 0.05 and 0.03 ± 0.05 logMAR for distance photopic, 0.16 ± 0.10 and 0.23 ± 0.07 logMAR for distance mesopic and -0.03 ± 0.05 and 0.05 ± 0.06 logMAR for near photopic, respectively. Average binocular spectacle corrected and PM VA were -0.05 ± 0.07 and -0.01 ± 0.03 logMAR for distance photopic, 0.10 ± 0.06 and 0.18 ± 0.05 logMAR for distance mesopic and -0.08 ± 0.06 and 0.02 ± 0.05 logMAR for near photopic, respectively. Statistically significant differences were found between the contrast sensitivity with the two types of correction for both distance and near, with spectacle correction being better in all cases.

CONCLUSION

  PureVision multifocal contact lenses preserve good VA and good visual performance under real-life conditions.

Effects of age on dynamic accommodation

Visual accommodation plays a critical role in one's visual perception and activities of daily living. Age-related accommodation loss poses an increased risk to older adults' safety and independence. Although extensive effort has been made towards understanding the effect of age on steady-state accommodation, dynamic aspects of accommodation is still unknown. A study was therefore conducted to investigate age-related dynamic accommodative characteristics utilising a modified autorefractor. Ten individuals from each of three age groups (i.e. younger group: 20 to 29 years old; middle-aged group: 40 to 49 years old; older group: 60 to 69 years old) were recruited and their dynamic accommodation responses were examined. The laboratory experiment was designed to assess dynamic accommodation associated with an abrupt change from a constant far target (400 cm, 50 cd/m(2)) to a near target (70 cm, 100 cd/m(2) or 20 cd/m(2)), which aimed to simulate car dashboard reading behaviour while driving. The results of the study indicated that age and target intensity both had a significant impact on dynamic accommodation. These effects were attributed to both the age-related physiological limitation of the eye as well as to central neural processing delay. A method of measuring dynamic accommodation and the implications of the study are discussed. STATEMENT OF RELEVANCE: The results of the study indicate that age and target intensity both have a significant impact on dynamic accommodation. These effects are attributed to age-related physiological limitation of the eye as well as central neural processing delay and to decreased sensitivity of the cone photoreceptors. To enhance the visual performance of the ageing population involving dynamic accommodation, target distance and target light intensity should be carefully evaluated to facilitate effective viewing.

Age-related changes in static accommodation and accommodative miosis

An attempt was made to explore the validity of the Hess-Gullstrand and Duane-Fincham models of presbyopia development, on the assumption that accommodative miosis could be used as an indicator of ciliary muscle effort. Monocular accommodation response and pupil size were measured as a function of accommodation demand over the range 0-4 D, in 48 normal subjects with ages between 17 and 56 years. The slope of the response/stimulus curve was found to decrease only slowly with age up to about 35 years and then to decline more rapidly. Accommodative miosis per dioptre of accommodation response did not change systematically with age up to about 35 years, this being apparently more in accord with the Hess-Gullstrand model. However, accommodative miosis varied very widely between younger subjects of similar age and accommodative amplitude (from zero to around 1 mm per dioptre of accommodation response for subjects in their twenties). It is concluded that miosis does not necessarily accompany accommodation and that its magnitude is not related in any simple general way to ciliary muscle contraction. Hence it cannot be used to support or refute particular theories of presbyopia.

On the relationship between lens stiffness and accommodative amplitude

The purpose of this study was to investigate the relationship between the stiffness of the material comprising the lens and the loss of accommodative amplitude with age. We used a validated mechanical model to determine the changes in the shape of the lens during accommodation and disaccommodation. The relative contribution of lens stiffness to loss of accommodative amplitude with age was determined by varying lens stiffness in the model. The changes in lens stiffness with age were based on the results of two recently published studies. In the first study we showed that lens stiffness increases exponentially with age, and in the second study we showed that there is a considerable stiffness gradient within the lens that changes with age. The results of both studies were incorporated in the mechanical model. The model showed that it is not the increasing stiffness of the lens with age, but rather the changing stiffness gradient that influences accommodative amplitude. The results show that the changing stiffness gradient in the lens may be responsible for almost the entire loss of accommodation with age.

Retinal image quality, reading and myopia

Analysis was undertaken of the retinal image characteristics of the best-spectacle corrected eyes of progressing myopes (n = 20, mean age = 22 years; mean spherical equivalent = -3.84 D) and a control group of emmetropes (n = 20, mean age = 23 years; mean spherical equivalent = 0.00 D) before and after a 2h reading task. Retinal image quality was calculated based upon wavefront measurements taken with a Hartmann-Shack sensor with fixation on both a far (5.5 m) and near (individual reading distance) target. The visual Strehl ratio based on the optical transfer function (VSOTF) was significantly worse for the myopes prior to reading for both the far (p = 0.01) and near (p = 0.03) conditions. The myopic group showed significant reductions in various aspects of retinal image quality compared with the emmetropes, involving components of the modulation transfer function, phase transfer function and point spread function, often along the vertical meridian of the eye. The depth of focus of the myopes (0.54 D) was larger (p = 0.02) than the emmetropes (0.42 D) and the distribution of refractive power (away from optimal sphero-cylinder) was greater in the myopic eyes (variance of distributions p < 0.05). We found evidence that the lead and lag of accommodation are influenced by the higher order aberrations of the eye (e.g. significant correlations between lead/lag and the peak of the visual Strehl ratio based on the MTF). This could indicate that the higher accommodation lags seen in myopes are providing optimized retinal image characteristics. The interaction between low and high order aberrations of the eye play a significant role in reducing the retinal image quality of myopic eyes compared with emmetropes.

A retrospective study on presbyopia onset and progression in a Hispanic population

BACKGROUND

This study evaluates differences in age and presbyopia progression between an ethnic Hispanic and a non-Hispanic patient population.

METHODS

Patient records from the Optometric Center of Los Angeles were examined retrospectively from 1998 through 2001. The first part of the study compared ages at onset and progression of presbyopia as a function of ethnicity. The second part of the study compared the amplitude of accommodation for the cohort of patients to Hofstetter's expected norms.

RESULTS

A total of 332 patient records were evaluated: 61% (n = 203) Hispanic and 39% (n = 129) non-Hispanic. For an add power of +0.75 D, presbyopia developed in the reviewed Hispanic population at 39.31 years of age in comparison to development in non-Hispanics at 40.22 years. Hispanics also had a similar progression of reading add power (0.105 D/year) as compared to non-Hispanics (0.097 D/year) (t= 0.798, p = 0.43). The progression of amplitude of accommodation for the Hispanic (amplitude = 11.2 - 0.132 x age) and non-Hispanic populations (amplitude = 9.72 - 0.18 x age) (t= 0.0997, p = 0.92) were comparable, but both groups show a slower decline of amplitude when compared to Hofstetter's norms (p< 0.001).

CONCLUSIONS

No statistically significant difference in the age at onset and progression of presbyopia was found between Hispanic and non-Hispanic patients; however, both groups of patients have a later onset and slower progression of presbyopia when compared to Hofstetter's norms.

Accommodation as a function of age and the linearity of the response dynamics

The changes with age in the accommodation responses to dynamic stimuli can reveal useful information on the factors underlying presbyopia development. Analysis of the monocular accommodation responses of 19 normal observers (ages 18-49 years) to stimuli whose vergence varied sinusoidally with time at different temporal frequencies (peak-to-peak stimulus 1.33-2.38 D, at 0.05-1.00 Hz) showed that at all ages both the gain and phase of the response were essentially linear functions of the temporal frequency. Extrapolation of least-squares, regression line fits to the gain data for each subject gave the gain at zero frequency, G0, and the cut-off frequency, fc, at which the gain fell to zero. G0 reduced with age but fc remained essentially constant at about 1.7 Hz, up to at least the age of about 40. The magnitude of the response to step stimuli covering the same stimulus range was well correlated with the value of G0. The linear changes in phase lag with temporal frequency corresponded to simple time delays td. The time lag varied from close to zero for the youngest subjects to about 0.5 s for the subjects in their late forties. There was substantial variation between the responses of subjects of similar age: those subjects with high values of G0 also tended to have low values of td, both effects probably being due to the superior ability of some individuals to predict the sinusoidal changes in the accommodation stimulus. Comparison of theoretical step responses, derived by applying linear theory to the parameters obtained from the results for the sinusoidal stimuli, with the actual responses to unpredictable steps for the same subjects supports the view that prediction effects and other possible factors make linear theory inapplicable to this type of data. The results are discussed in the context of current ideas on the development of presbyopia: it is suggested that the constancy of fc with age is the result of the ciliary body maintaining its efficiency, whereas the fall in G0 and increase in td result from increases in lens rigidity.

Accommodation dynamics as a function of age

Experiments are described in which the dynamic accommodation response to a stimulus whose vergence was varying sinusoidally with time between 1.33 and 2.38 D was measured as a function of frequency for 19 subjects, with ages distributed between 18 and 49 years. Response to abrupt stimulus change between the same levels was also measured. The results showed that at any age, for the sinusoidal stimuli, gain fell with frequency and phase lag increased: at fixed frequency, gain fell with age and phase lag increased. Neither reaction nor response times for step stimuli changed with age. Analysis of the sinusoidal data suggests the possible existence of a cut-off frequency of about 2 Hz, which varies little with age and above which the system cannot respond. The phase data is compatible with the existence of a frequency-independent time delay, which increases with age from about 0.17 s at 20 years to 0.48 s at 40 years. The results are discussed in terms of current knowledge of the factors contributing to the development of presbyopia.

Are there age differences in the accommodative response curve between 3 and 14 years of age?

The monocular ocular accommodative responses of 118 emmetropic children, aged from 3 to 14 years, were measured at five different stimulus distances (0, 1, 2, 3, 4 D) from the principal plane of the eye with a Canon Autoref R-1. The change in the accommodative response curve in emmetropic children between 3 and 14 years old was found to be negligible.

Dynamics of the accommodation response to abrupt changes in target vergence as a function of age

The dynamic accommodation responses to small, abrupt changes in an accommodation stimulus were studied in two experiments. In the first, responses of 19 subjects with ages distributed between 18 and 49 years were measured for step stimuli of +/- 1.05 D. In the second, responses to small step stimuli (+/- 0.75, +/- 1.75 D) were recorded for a group of six 'young' (mean age 22, range 16-26 years) and six 'old' (mean age 42, range 36-48 years) subjects. In both experiments, the low target vergences always lay within the accommodation range of all subjects. Whether the data were analysed in terms of simple reaction and response times, or in terms of the frequency response, no marked systematic changes with age were found in the dynamics of the response in either of the experiments. It is concluded that, for small stimuli within the amplitude of accommodation, the response dynamics over the adult age range studied remain remarkably constant, even though the amplitude progressively reduces with age as complete presbyopia is approached.

Senescent changes in scotopic contrast sensitivity

Scotopic contrast sensitivity functions (CSFs) were measured for 50 observers between the ages of 20 and 88 years. Using a maximum-likelihood, 2-alternative, temporal forced-choice threshold-estimation algorithm, scotopic CSFs were measured at 7 spatial frequencies ranging from 0.2 to 3.0 cpd, with mean retinal illuminance equated for observers at -0.85 log scotopic Trolands. For each stimulus condition, eight cycles of a horizontal sinusoidal grating were presented within +/- 1 S.D. of a 2-D Gaussian-spatial envelope and within a 1-s Gaussian-temporal envelope. Stimuli were centered on the nasal retina along the horizontal meridian 6 degrees from the fovea. Scotopic CSFs were found to be low-pass. Statistically significant age-related declines in contrast sensitivities were found for spatial frequencies at or below 1.2 cpd. There was also a statistically significant decrease in the high frequency cut-off with age (P < 0.01). An explanation of these results in terms of optical factors is rejected, while the results are consistent with age-related changes in the magnocellular pathway.

Static aspects of accommodation: age and presbyopia

Although the progressive reduction in accommodative amplitude with increased age is well documented, little is known about several other aspects of static or steady-state accommodation to provide a comprehensive assessment of changes related to age and presbyopia. Static components of accommodation (tonic accommodation, depth-of-focus, slope of the stimulus/response function, and accommodative controller gain) were assessed objectively using an infrared (IR) optometer in 30 human subjects aged 21-50 years; depth-of-focus was also determined psychophysically as was accommodative amplitude. Tonic accommodation and the amplitude of accommodation decreased with increased age, whereas the subjective depth-of-focus increased; the other parameters remained unchanged. The decrease in tonic accommodation and amplitude of accommodation was attributed to biomechanical factors, whereas the increase in subjective depth-of-focus was believed to result from increased tolerance to defocus related to the gradual onset of presbyopia. Constancy of the objective depth-of-focus suggested absence of age effects on the neurologic control of reflex accommodation, whereas the lack of systematic change in slope and controller gain provided support for the Hess-Gullstrand theory of accommodation and presbyopia.

Sensitivity analysis of the stimulus-response function of a static nonlinear accommodation model

The effect of parameter variation of a nonlinear static feedback control model of the accommodation system was investigated. Simulations of a MATLAB/SIMULINK model showed a nonlinear relationship between stimulus and response in which the response curve was above the 1:1 line in the region to the left of the crossover and below the 1:1 line in the region to the right of the crossover. At the crossover, the response curve exhibited an inflection that was constant and equal to the tonic accommodation value (ABIAS). Sensitivity analysis showed that increasing depth of focus (deadspace range between +/- DSP) increased the separation between the boundary lines of the deadspace region, with a larger separation associated with late-onset myopia and congenital nystagmus. Increasing accommodative controller gain (ACG) increased the slope of the function on either side of the deadspace, with lower ACG values corresponding to lower slopes that indicated an amblyopic deficit. Increasing ABIAS increased the accommodative level at the inflection region. In addition, the saturation level of the accommodative response decreased with increasing age, while the slope remained the same, which was consistent with the Hess-Gullstrand theory of presbyopia. The accuracy and relative simplicity of the model indicated that it could serve as a basis for further comprehensive investigation of the basic and clinical aspects of the accommodation system.

Tonic accommodation: a review. II. Accommodative adaptation and clinical aspects

Part I of this review considered basic aspects of tonic accommodation (TA), i.e. the accommodative response observed under degraded stimulus conditions. Part II considers accommodative adaptation, i.e. the apparent change in TA following periods of sustained fixation, and clinical aspects of both baseline TA and accommodative adaptation. It is suggested that the apparent post-task shift in TA reflects the slow rate of decay of the stimulus-mediated adaptive accommodative response, while the actual level of tonic innervation to the ciliary muscle remains relatively constant. The clinical implications of both TA and accommodative adaptation are discussed with regard to night, space and instrument myopia and refractive error development, notably nearwork-induced myopia. It is concluded that the evidence for any association between this form of myopia and either TA or accommodative adaptation is equivocal, and furthermore it seems likely that TA plays only a minor role in influencing the closed-loop steady-state accommodative response.

Tonic accommodation: a review. I. Basic aspects

In the absence of an adequate visual stimulus, accommodation adopts an intermediate position of approximately 1 D. Since this position was believed to reflect the level of tonic innervation to the ciliary muscle, this response has been termed tonic accommodation (TA). Part I of this review will consider various aspects of this parameter, including its reference to closed-loop accommodative function and autonomic physiology. In addition, both the methods of measurement and appropriate terminology for this function will be discussed. It is concluded that the response, which becomes apparent under so-called 'stimulus-free' conditions, in fact probably represents an aggregate response resulting from multiple, non-optical stimuli. Thus the designation tonic accommodation may not be appropriate, since it fails to describe accurately the heterogeneous composition of the stimulus-free accommodative response. An associated paper (to be published as part II of this review) will examine accommodative adaptation and both clinical aspects of TA and adaptation of TA.